Here is my dilemma. I have seen a lot of videos of airfoils in wind tunnels, and I've noticed how the airflow always moves away from the trailing edge parallel to the wing, or at a very small angle away from the wing (link to one of those videos

). I've also heard that propellers are nothing more than rotating wings (airfoils), producing lift sideways as opposed to upwards. What confuses me is that in the case of the propeller, the airflow leaves the propeller at a much higher angle, and not straight as is with the wing. In other words, the airflow is deflected backwards from the prop. I just can't understand how the wing and propeller, which are two very similar shapes (maybe not that similar, but they operate by the same principle), deflect air in two radically different directions.

I have a few thoughts non why this happens, but I think it's better if someone tells my first.

5 Answers
5

On a stationary aircraft the propeller keeps beating the air at the same spot. That generates a horizontal column of air. If you took wings lifted them more than 100 feet off the ground and kept beating the same spot you'd also get a column of air moving. Actually that precisely describes what a helicopter does. A helicopter has "wings" oriented horizontally and rotating fast. It also has a tail rotor (wings) oriented vertically.

A wing does push air down but it doesn't generate a column of air moving simply because it always keeps moving on to a different air mass. Same thing with a propeller moving fast forward. It doesn't actually move the air backwards very much. It also moves on to new air.

Expanding on @RainerP's answer, look more carefully at the wind tunnel footage. The smoke streams are slightly lower behind the wing than they are in front of it. This may look insignificant, but it isn't: the air in the tunnel is moving extremely quickly, and therefore only has a very short length of time to move vertically after it has left the wing before it is carried off camera by its horizontal motion... if we can see a 3m section of tunnel and the air is moving at 200mph, then it stays on camera for only around 30ms. So if the air coming off the wing gains a vertical velocity of, say, 3 meters per second, you'd only expect to see a deflection of around 10cm by the time it reaches the edge of the frame.

The airflow behind an aerofoil (sorry, I'm English, I can't bring myself to write 'airfoil'!) always moves downwards away from the trailing edge, if the aerofoil is generating lift. This is Newton's Third Law in action: every action has an equal and opposite reaction. It is not possible for the air to act on the aerofoil to generate lift without the aerofoil acting on the air to accelerate it downwards.

From Newton's Second Law, the force required to accelerate something is proportional to the mass of the thing and the amount by which it's accelerated ($F=m\times a$), so to generate the same amount of reaction force you can accelerate a small amount of air a lot, or a large amount of air a bit. In a continuous process this force is also equal to the mass flow of the thing being accelerated (the mass per unit time) multiplied by its ultimate change in velocity.

Wings are typically bigger than propeller blades, so the mass flow of air past them is higher for the same airspeed, and the required change in velocity is therefore lower for a given amount of lift.

(Side note: it might appear that propeller blades have a much higher airspeed than wings, but in flight they really don't. The tips are almost always subsonic, and the linear speed of the blade decreases towards the hub too, so the speeds involved are loosely comparable to those of a wing.)

It sounds like you might be confusing the direction of airflow vs. the direction of lift: As either a wing or a propeller moves through the air, the flow is approximately parallel to the airfoil. In both cases, the air at the trailing edge of the airfoil is moving "downward" (relative to the airfoil) to some extent. This is what produces lift (on a wing) and thrust (on a propeller), both of which are approximately perpendicular to the airfoil. For a given air-relative speed and airfoil shape, both a wing and a propeller will deflect the airflow the same amount: A propeller does not deflect the air perpendcular to the rotor, it provides thrust in that direction.

As an airplane moves through the air, its wings (hopefully) hold it the same distance off the ground. As a propeller (on a moving airplane) moves through the air, the path it follows looks much like a screw, with each pass of the propeller being roughly the same distance from the previous path.